The use of phosphating compositions for inhibiting corrosion on metal substrates and improving the adhesion of superimposed organic coatings, e.g., paints and lacquers, is an old and crowded art.
Phosphating compositions typically applied by immersion of the product to be coated in a bath solution or by spraying, commonly have been in the form of acidic, aqueous solutions typically containing phosphate ions, an oxidizing agent, and divalent, layer-forming metal cations. The layer-forming ions most commonly included zinc ions used alone or in combination with the ions of other metals.
In U.S. Pat. No. 3,597,283 to Charles T. Snee, the use of zinc ion in the amount 0.5-3.0 grams per liter is combined with nickel ion, 0.05-3.0 grams per liter, or cobalt ion, 0.003-0.7 grams per liter, or copper ion, 0.003-0.7 grams per liter, and magnesium ion 1-8 grams per liter. The presence of magnesium is recited to be essential.
In European Patent Application Publication No. 0060716 assigned to Nippon Paint KK, the use of zinc ion in the amount of 0.5-1.5 grams per liter is combined with 0.6-3 grams per liter of manganese ion. Optionally, the coating solution contains 0.1 to 4 grams per liter nickel ion.
In our U.S. Pat application Ser. No. 735,286 filed Jan. 6, 1984, we demonstrated that the resistance to alkaline dissolution of a zinc phosphate conversion coating on a corrodible metal surface and the corrosion resistance after painting is increased if nickel ions are employed in such a bath in an amount such that they comprise between about 84 and and 94 mole percent of the divalent metal cations in the bath, the remainder being essentially zinc ions.